Shu'bah said: Muhammad bin al-Munkadir asked me: What is your name? I said: Shu'bah. He said: Abu Shu'bah narrated to me from Sayyiduna Suwayd bin Muqarrin al-Muzani (may Allah be pleased with him) that he saw a man slapping his slave on the face, and said: Do you not know that violating (the dignity of) the face is forbidden? I have seen myself as the seventh of seven brothers in the time of the Messenger of Allah (peace and blessings be upon him), and we had only one servant. One of us slapped him on the face, and the Messenger of Allah (peace and blessings be upon him) ordered that we free him.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6423